1. Field of the Invention
The invention pertains to modulators for use in transmitting digitized multiplexed audio channels particularly for use in a microwave or satellite communication system. The invention further relates to a receiver and demodulator for receiving such multiplexed signals having reduced bandwidth and power requirements.
2. Description of the Prior Art
Microwave transmission systems and satellite relay systems have been finding increasing use in long-range communications. All types of data may be sent and received using these systems. Most frequently, the information to be transmitted is an audio or video signal which has been sampled and digitized prior to transmission.
For each channel of information to be transmitted, be it audio or video, a stream of digital words representing samples of the analog signal is produced which is usually in the form of a sequence of digital words, each of which includes a number of parallel or simultaneously presented binary bits. Eight bits per word or sample is the most common arrangement although other numbers have been used as well.
Within the stream of digital words for each channel, some of the words are service in addition to the data words which make up the majority of the total words to be transmitted. In the systems of the prior art, both the service words and the data words were transmitted using the same modulation technique. The well-known four-phase shift keying modulation technique (QPSK) was the most commonly employed modulation method. Amplitude modulation has also been employed.
With the QPSK technique, an output transmitted pulse is produced for each service or data word in the incoming data stream with the amplitude of the pulses constant independent of whether the word to which the pulse corresponded was a service word and whether or not any actual information was then present in the original data analog signal. Thus, even if there was absolutely no signal present on an incoming line, such as the silent periods which occupy up to 60% of an ordinary telephone conversation, the transmitted carrier was on at full power although no information was actually being conveyed by the output transmitted signal.
A number of different modulation techniques have been proposed for reducing the wasted transmitter power during times when no signal information is present in the corresponding analog input channel although none of these has met with wide acceptance. One reason that the newly proposed techniques have not met with wide acceptance would appear to be the fact that none recognizes that the service words must be transmitted with a lower bit error rate than may be tolerated for the data words which they accompany. By using only a single modulation technique for both service and data words, it has heretofore not been possible to minimize the transmitter power requirements while maintaining the necessary minimum levels of bit error rate for both service and data words.
In a still further attempt to conserve transmitter power, a known prior art system employed complicated techniques for speech compression and multiplexing so that no pulse signals were transmitted representing a silent voice channel within a multi-channel multiplexed system. Although this system did result in some savings of power and bandwidth, complicated and expensive voice detectors, the provision of unique identification words, and additional detectors and synchronizers were required.